At VHF frequencies, it is possible to achieve acceptable direction finding accuracy using an interferometer comprising two dipole elements spaced from each other by one wavelength. However, at HF frequencies, i.e. frequencies in the range of approximately 2-30 Mhz., interferometry is not a practical way to achieve highly accurate direction finding, in part because the required element spacing is prohibitively large. Most other direction finding antennas usable in the HF range are comparatively inaccurate.
One type of device which is able to achieve high accuracy in the HF range is the field probe. A field probe is a device which measures one or more components of an electromagnetic field. Direction finding, by means of a field probe, is fundamentally a process of comparing amplitudes of field components, as detected by two or more antennae having different antenna patterns.
A time-varying electromagnetic field is completely characterized at any point in space, in Cartesian coordinates, by six vector quantities: the electric field parallel to the x axis (E.sub.x), the electric field parallel to the y axis (E.sub.y), the electric field parallel to the z axis (E.sub.z), the magnetic field parallel to the x axis (H.sub.x), the magnetic field parallel to the y axis (H.sub.y), and the magnetic field parallel to the z axis (H.sub.z).
An electromagnetically complete field probe resolves the electric and magnetic field components of an electromagnetic wave into their orthogonal components E.sub.x, E.sub.y, E.sub.z, H.sub.x, H.sub.y and H.sub.z. By applying known, model-based "superresolution" algorithms to the output of the field probe, direction finding accuracies comparable to those achievable by interferometry can be realized.
Preferably, in the interest of transportability and cost, and in the interest of measurement accuracy as well, the field probe is electrically small, i.e. its element lengths and spacings are only small fraction of the wavelength of the received signal. An electrically small antenna is inefficient. However, in the HF range, external noise, rather than internal noise, is the primary noise problem. Since external noise and signal strength vary together in dependence on antenna efficiency, it is possible to compensate for the inefficiency of a small antenna by amplification, without seriously affecting the signal to noise ratio (S/N). Thus, the size of an electromagnetic field probe can be reduced until a point is reached at which there is a serious degradation in the overall signal to noise ratio. A practical probe suitable for use in the HF range can have a maximum dimension no more than approximately two meters.
One form of prior art electromagnetically complete field probe comprises a three mutually perpendicular dipoles and three mutually perpendicular loops, all symmetrical about a common center point. The dipoles measure the electric field components E.sub.x, E.sub.y and E.sub.z, while the loops measure the magnetic field components H.sub.x, H.sub.y and H.sub.z.
Each dipole is in a special configuration and comprises an intermediate conductor extending through the center of the array, and two end conductors, one on each end of the array. For example, the dipole for measuring E.sub.x has an intermediate conductor and two end conductors, all extending along the x axis. The end conductors are separated from the intermediate conductors by gaps, and electrical connections are from the gaps, through a four-port hybrid junction to an x-axis electric field (E.sub.x) output port. The E.sub.y and E.sub.z dipoles are similarly constructed and connected to output ports.
The loops are situated respectively in the y-z, x-z and x-y planes. Each loop comprises four elements, each element corresponding to the side of a rectangle. Electrical connections are made to the elements at gaps formed at the corners of the rectangle. The electrical connections for each loop are made, through an array of four-port hybrid junctions to an output port. Thus the loop in the y-z plane is connected to an H.sub.x output port; the loop in the x-z plane is connected to an H.sub.y output port; the loop in the x-y plane is connected to an H.sub.z output port.
Although the electromagnetically complete field probe is used as a receiving antenna, its operation can be most easily understood by treating each dipole and loop as a transmitting antenna.
A voltage applied to the x-axis electric field port would induce a current distribution in the intermediate conductor and end conductors of the x-axis dipole. Similar voltages applied to the y-axis and z-axis electric field ports would induce current distributions in the corresponding dipoles. Thus, by the theorem of electromagnetic reciprocity, when the probe is subjected to an incident electromagnetic field, the electric field output ports provide voltages proportional to the electric field components along the axes of their corresponding dipoles.
In a similar way, a voltage applied to the x-axis magnetic field port (H.sub.x) would excite voltages at the corners of the loop in the y-z plane which induce a current distribution in the elements of the loop. Likewise, a voltage applied to the y-axis magnetic field port (H.sub.y) would induce a current distribution in the elements of the loop in the x-z plane, and a voltage applied to the z-axis magnetic field port (H.sub.z) would induce a current distribution in the elements of the loop in the y-z plane.
The arrangement of elements in the electromagnetically complete field probe substantially eliminates the effects of mutual coupling between elements. For example, viewing the loops and dipoles of the device once more as transmitting antennae, if the loop in the x-y plane is excited, currents would be induced in elements of the x and y-axis dipoles. However, because the loop is excited at each of its corners, the current distribution in the loop is nearly balanced. Therefore, in each element of the x and y axis dipoles, currents induced by mutual coupling with the loop in the x-y plane oppose and cancel one another.
One of the problems with the prior field probe, as described above, is that it is physically difficult to connect the magnetic field ports to the corners of the three loops. Another problem is that, in the transmitting antenna model of the device, exact balancing and cancellation of induced currents is not achieved. Correspondingly, in the field probe, unbalanced conditions exist, which adversely affect the accuracy of measurement.